Hundreds of youths clashed with Egyptian police in Tahrir Square this morning in a violent start to the second anniversary of the uprising that toppled Hosni Mubarak and led to the election of an Islamist president who is now the focus of rage.

Opponents of President Mohamed Mursi and his Muslim Brotherhood allies are expected to mass in Tahrir Square later on Friday to revive the demands of a revolution that they say has been betrayed by the Islamists.

The square was calm by daybreak, following early morning battles between police and protesters who threw petrol bombs and firecrackers as they tried to approach a wall blocking access to government buildings near the square. Plumes of teargas fired by the police filled the air.

Egyptian protesters clashed with police on eve of the second anniversary of the uprising, which toppled longtime authoritarian president Hosni Mubarak in 2011

Protesters use a metal pole to try to tear down a cement wall built to prevent them from reaching parliament and the Cabinet building near Tahrir Square

Anger: The protesters tear down a cement wall built to prevent them from reaching the Egyptian parliament and Cabinet building near Cairo's Tahrir Square

The Health Ministry said 16 people had been wounded. At one point, riot police used one of the incendiaries thrown at them to set ablaze at least two tents erected by the youths, a Reuters witness said.

Inspired by Tunisia's uprising against President Zine al-Abidine Ben Ali, Egypt's revolution helped set off more revolts in Libya and Syria. But the sense of common purpose that united Egyptians at the time has given way to conflict that has grown only worse and last month triggered lethal street battles.

The anniversary will once again showcase the divide between the Islamists and their secular opponents. The Brotherhood has decided against mobilising in the street for the occasion, a decision that could reduce the likelihood of confrontation.

Egyptian demonstrators wave the national flag as others shout slogans during a protest in Tahrir Square today

A man prepares to hurl a brick at riot police in Tahrir Square. Huge crowds are expected to demonstrate today on the second anniversary of the revolution that brought in an Islamist government which some feel has betrayed them

Casualty: A masked protester carries a young boy injured during violent clashes on Thursday afternoon

Egyptians carry a wounded protestor during clashes with riot police in Tahrir Square

The euphoria that followed the 2011 revolution has been replaced by a growing anger and frustration

Uprising: A protester wearing a Guy Fawkes mask clutches a riot shield and a woman covers her face as she flees from teargas

'The people want to bring down the regime,' declared banners in the square. 'Save Egypt from the rule of the Supreme Guide,' said another, a reference to leader of the Muslim Brotherhood, Mohamed Badie.

Mursi, in a speech on Thursday marking the Prophet Mohammad's birthday, called on Egyptians to mark the anniversary 'in a civilised, peaceful way that safeguards our nation, our institutions, our lives'.

Many Egyptians feel let down by President Mohamed Mursi and his Muslim Brotherhood and say he has betrayed the revolution

'The Brotherhood is very concerned about escalation, that's why they have tried to dial down their role on Jan. 25,' said Shadi Hamid director of research at the Brookings Doha Center.

'There may very well be the kinds of clashes that we've seen before, but I don't see anything major happening that is going to fundamentally change the political situation,' he said.

Mursi faces discontent on multiple fronts.

His opponents say he and his group are seeking to dominate the post-Mubarak order. They accuse him of showing some of the autocratic impulses of the deposed leader by, for example, driving through a controversial new constitution last month.

The Brotherhood dismisses such criticism as unfair. It accuses its opponents of failing to respect the rules of the new democracy that put the Islamists in the driving seat by winning elections.

Six months into office, Mursi is also being held responsible for an economic crisis caused by two years of turmoil. The Egyptian pound has sunk to record lows against the dollar.

Other sources of friction abound. Activists are impatient for justice for the victims of political violence perpetrated over the last two years.

Casualty: A wounded protestor is carried away following clashes with riot police in Tahrir Square yesterday

Blockade: Egyptian anti-riot police block the entrance to the presidential palace in Cairo

Scatter: Protesters run for cover following clashes with riot police in Tahrir Square

A masked protestor, with a stone clasped in his hand, takes a breather during clashes yesterday afternoon

Little has been done to reform brutal Mubarak-era security agencies. A spate of transport disasters on roads and railways neglected for years is feeding discontent.

The parties that have called for Friday's protest list demands including a complete overhaul of the Islamist-tinged constitution that was fast-tracked into law by Mursi in December, a move that fuelled street violence.

Its critics say the constitution, which was approved in a popular referendum, offers inadequate protection for human rights, gives the president too many privileges and fails to curb the power of the military establishment.

A protester brandishes a home-made pistol whole another prepares to throw a Molotov cocktail over a wall

Mursi's supporters say the criticism is unfair, that enacting the constitution quickly was crucial to restoring stability, and that the opposition is making the situation worse by perpetuating unrest.

With its eye firmly on forthcoming parliamentary elections, the Brotherhood is marking the anniversary with a big charity campaign. It aims to deliver medical aid to 1 million people, offer affordable basic foods, and renovate some 2,000 schools.

A strong turnout on Friday could also help the Brotherhood's opponents ahead of the elections. 'There is a lot of power in this day and a real chance to use that to mobilise their supporters in the lead-up to the elections,' Hamid said.